Recently regulations on exhaust gas in a vehicle for the purpose of protection on the global environment have been increasingly tightened annually. There has been proposed the associated technique which acquires information on oxygen concentration in the exhaust gas or the like and uses the acquired information for various controls. For example, there are studies on techniques of correcting variations in fuel injection quantity and EGR quantity, and detecting a deterioration state of an exhaust gas purifying device such as a catalyst, based upon information on oxygen concentration in the exhaust gas.
A known gas sensor for acquiring information on oxygen concentration in the exhaust gas is provided with a sensor element formed of solid electrolyte such as zirconia. The sensor element is maintained at a predetermined active temperature, thereby outputting a density detection signal in accordance with the then-oxygen concentration. In this case, the sensor element houses a heater therein. When the heater is energized to generate heat, a temperature of the sensor element rises and the sensor element is maintained at an active state.
A gas sensor is generally installed in an exhaust pipe connected to an engine body. Therefore, when at cold startup of an engine, water vapor in the exhaust gas is condensed to form condensed water, the condensed water threatens to be attached to the sensor element which is being heated by the heater. When the condensed water is attached to the sensor element during heating, excessive heat stress acts on the sensor element, bringing about the likelihood that the sensor element is damaged. When the sensor element is damaged, the sensor output deteriorates in accuracy, resulting in the possibility that adverse affect be applied to a control system for an engine or the like.
Therefore, there is proposed a technique for controlling power supply to the heater after the engine startup for protection of the sensor element or the like. For example, JP-A-9-184443 discloses the technique that in an exhaust system with a catalyst, an accumulated heat quantity introduced to the catalyst after the engine startup is calculated and also the power supply to the heater is designed to be prohibited until the accumulated heat quantity reaches a predetermined value. In addition, JP-A-2003-83152 discloses the technique that it is determined whether or not water components are attached to an inner wall of an exhaust pipe. When the water components are attached to the wall, the power supply to the heater is controlled in such a way as to maintain the heater at a temperature lower than usual. A determination on attachment of the water components to the inner wall of the exhaust pipe is made based upon an exhaust pipe temperature calculated by an exhaust gas temperature and an outside air temperature.
However, in the case of JP-A-9-184443, even if the power supply to the heater is allowed based upon the introduced heat quantity to the catalyst after the engine startup, there is the possibility that the condensed water is in fact present at a sensor location part of the exhaust pipe at the allowance timing. In addition, in contrast, there is the possibility that the condensed water at the sensor location part of the exhaust pipe already dries out before allowance of the power supply to the heater (a state of power supply prohibition). This case raises the problem that the power supply to the heater cannot start at proper timing and therefore, desired protection or earlier activation of the sensor element cannot be realized.
Further, in the case of JP-A-2003-83152, the attachment of the water components to the inner wall of the exhaust pipe is estimated based upon the temperature of the exhaust pipe calculated from the exhaust gas temperature and the outside air temperature. However, in the event of application of a gas sensor to an actual vehicle, even if the temperature of the exhaust pipe is estimated by using the exhaust gas temperature and the outside air temperature as parameters as described above, the estimation accuracy results in a low state. This is because a correlation between an exhaust gas temperature and an exhaust pipe temperature varies with an engine operating condition or a vehicle driving condition. Therefore, JP-A-2003-83152 has the possibility that the power supply to the heater cannot be managed appropriately.